1. Field of the Invention
The present invention relates to image forming apparatus such as copying machine, facsimile, printer, etc. and, in particular, to apparatus in which a color image, images of different colors are superimposed, is formed and borne on a recording material and the recording material is heated and pressurized by an image fixing means so as to fix the color image thereon.
2. Related Background Art
As fixing apparatus (fixing devices) used in image forming apparatus for fixing an unfixed toner image, which is indirectly (by means of transferring) or directly formed and borne on a recording material (paper) by a proper image forming process means such as electrophotographic process, on the surface of the recording material as a permanent fixed image by the application of heat, those adopting the heat roller fixing method have been widely in use.
In recent years, there have been put to practical use apparatus adopting the heater type film heating fixing method from the standpoint of quick start and saving energy. Further, there have been proposed apparatus adopting the electromagnetic induction film heating fixing method which causes a film of metal itself heat generation.
a) Fixing Apparatus adopting the Heater Type Film Heating Fixing Method
There are disclosed fixing apparatus adopting the film heating fixing method in, for example, Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, Japanese Patent Application Laid-Open No. 4-44075, and Japanese Patent Application Laid-Open No. 4-204980.
Specifically, these fixing apparatus are such that they have a pressure contacting nip portion (hereinafter referred to as fixing nip portion) formed therein by sandwiching a heat-resistant film (hereinafter referred to as fixing film) between a ceramic heater as a heating body and a pressurizing roller as a pressurizing member, introduce a recording material having an unfixed toner image formed and borne thereon between the fixing film and the pressurizing roller at the above fixing nip portion, convey the recording material together with the fixing film while allowing the same to be nipped between the ceramic heater and the pressurizing roller so as to apply pressuring force of the fixing nip portion to the recording material while providing the same with heat of the ceramic heater via the fixing film, thereby fix the unfixed toner image on the surface of the recording material.
The fixing apparatus adopting the film heating fixing method offer the advantages that they can constitute an on-demand type fixing apparatus using a member with a low heat capacity for the ceramic heater and the film, they should apply current to the ceramic heater as a heat source only when executing image formation to cause the ceramic heater heat generation at a predetermined temperature, their latency from the instance of turning on them until they are in state where they can execute image formation is short (a quick start characteristic), and that they have a substantially low power consumption when standing by (power saving).
b) Fixing Apparatus adopting the Electromagnetic Induction Film Heating Fixing Method
There is disclosed in Japanese Utility Model Application Laid-Open No. 51-109739 a fixing apparatus adopting the electromagnetic induction film heating fixing method which induces an eddy current in a metal layer (heat generating layer) of the fixing film with magnetic flux and causes the metal layer Joule heat generation. This fixing apparatus enables the direct heat generation of the fixing film utilizing the generation of an induced current and has accomplished a highly efficient fixing process compared with the fixing apparatus adopting the heat roller fixing method which utilizes a halogen lump as a heat source.
Depending on the arrangement of coils and a core, however, since the energy of the alternating magnetic flux generated by excitation coils as magnetic field generating means is used for heating the entire fixing film, the radiation loss is big. As a result, there have been cases where the rate of the making energy applied to the fixing process is low, and hence, an inefficient fixing process.
Thus, there has been provided a highly efficient fixing apparatus in which excitation coils are arranged closer to a fixing film, as a heat generating element, and the alternating magnetic flux distribution of the excitation coils is concentrated on the vicinity of the fixing nip portion, so as to obtain the energy to be applied to the fixing more efficiently.
FIG. 16 is a schematic view of the construction of one example of fixing apparatus adopting the electromagnetic induction film heating fixing method which has increased the fixing efficiency by concentrating the alternating magnetic flux distribution of the excitation coils on the fixing nip portion.
In the same figure, reference numeral 10 denotes a cylindrical fixing film, as a rotary body, including electromagnetic induction heat generating layers (conductive material layer, magnetic material layer, resistor layer).
Reference numeral 16 denotes a gutter-shaped film guide member of which cross section is almost semicircular, and the cylindrical fixing film 10 is loose-fit on the outside of this film guide member 16.
Reference numeral 15 denotes a magnetic field generating means placed on the inside of the film guide member 16, which consists of excitation coils 18 and an E-shaped magnetic core (core material) 17.
Reference numeral 30 denotes an elastic pressurizing roller, which forms a fixing nip portion N having a predetermined width in combination with the bottom of the film guide member 16 with a predetermined pressure contacting force while nipping the fixing film 10 at the fixing nip portion and allowing the same to mutually come in pressure contact with the elastic pressurizing roller and the bottom of the film guide member.
The magnetic core 17 of the above magnetic filed generating means 15 is placed in such a manner as to correspond to the fixing nip portion N.
The pressurizing roller 30 is rotatively driven by a driving means M in the counterclockwise direction shown by an arrow. When rotatively driving the pressurizing roller 30, torque acts on the fixing film 10 by the friction force generated between the above pressurizing roller 30 and the external surface of the fixing film 10; consequently, the above fixing film 10 is rotated around the periphery of the film guide member 16 at a peripheral speed almost corresponding to that of the pressurizing roller 30 in the clockwise direction shown by an arrow with its internal surface closely touching and sliding on the bottom of the film guide member 16 at the fixing nip portion N (pressuring roller drive fixing method).
The film guide member 16 serves to pressurize the fixing nip portion N, support the excitation coils 18 and the magnetic core 17, as a magnetic field generating means 15, support the fixing film 10, and stabilize the conveyance of the above film 10 during the rotation thereof. This film guide member 16 is an insulating member which does not interfere with magnetic flux""s passing and a material is used for it which can resist heavy loads.
The excitation coils 18 generate alternating magnetic flux with alternating current supplied from an excitation circuit not shown in the figure. The alternating magnetic flux distributes intensively at the fixing nip portion N due to the E-shaped magnetic core 17 placed in such a manner as to correspond to the position of the fixing nip portion N and generates an eddy current in the electromagnetic induction heat generating layers of the fixing film 10 at the fixing nip portion N. This eddy current generates Joule heat due to the resistivity of the electromagnetic induction heat generating layers. The electromagnetic induction heat generation of the fixing film 10 occurs intensively at the fixing nip portion N where alternating magnetic flux is allowed to distribute intensively, the fixing nip portion N is thereby heated at a high efficiency.
The temperature of the fixing nip portion N is controlled to keep the portion at a fixed temperature by controlling the current supply to the excitation coils 18 with a temperature control system including a temperature detecting means not shown in the figure.
Thus, the pressurizing roller 30 is rotatively driven, and with the rotational motion of the pressurizing roller, the cylindrical fixing film 10 starts to rotate around the periphery of the film guide member 16, the electromagnetic induction heat generation is caused in the fixing film 10 by the feed from the excitation circuit to the excitation coils 18 as described above, and the fixing nip portion N is heated to a fixed temperature. In state where the temperature of the fixing nip portion N is being controlled, a recording material P, which is conveyed from the portion of an image forming means not shown in the figure and has an unfixed toner image t formed thereon, is introduced between the fixing film 10 and the pressurizing roller 30 at the fixing nip portion N with the image side facing up, that is, with the image side facing the surface of the fixing film. At the fixing nip portion N, the image side closely touches the external surface of the fixing film 10, and the recording material P is conveyed together with the fixing film 10 through the fixing nip portion N while being nipped between the fixing film 10 and the pressurizing roller 30. During the process of being conveyed together with the fixing film 10 through the fixing nip portion N while being nipped between the fixing film 10 and the pressurizing roller 30, the recording material P is heated due to the electromagnetic induction heat generation of the fixing film 10, and the unfixed toner image t is heat fixed on the recording material P. The recording material P having passed through the fixing nip portion N is separated from the external surface of the rotating fixing film 10 and conveyed to be discharged.
Now toners used in full color image forming apparatus and the process of fixing the same will be described.
Toners for use in the full color image forming process which adopts, for example, electophotographic process are required to exhibit excellent melting and color mixing characteristics when applying heat thereto, and preferably those having sharp melting characteristics, that is, those having a low softening point and a low melt viscosity are used.
The use of such sharp melting toners enables the broader range of color reproduction of reproduced matter, and color copies faithfully reproducing the original image can be obtained.
Such highly sharp melting color toners, however, have a strong affinity for a fixing roller or a fixing film, and they are apt to offset on the fixing roller or the fixing film during the fixing operation.
Particularly in a fixing apparatus of a color image forming apparatus, since more than one toner layers: magenta, cyan, yellow and black layers are formed on a recording material, their offsets are very likely to occur.
Now the problems attendant to the use of sharp melting toners will be described taking the case of a fixing apparatus adopting the heat roller fixing method. In such a fixing apparatus, releasing agents, such as silicone oil, have been applied to the fixing roller in order to promote the releasing tendency of toners therefrom.
In this image forming process, however, there have been created difficulties as follows.
In the currently used fixing system in which releasing agents such as oil are applied to the fixing roller, its body inevitably has a complex construction, which makes its maintenance difficult, and moreover, the application of oil gives rise to a problem of promoting decrease in life of the fixing roller.
With the diversification of needs for copy in recent years, and perhaps because of the ecology boom of today, the needs for the image forming process what is called the double-side copy, which forms images on both sides of a recording material and reduces the consumption of paper, are increasing day by day.
Under these circumstances, in the fixing system in current use in which releasing agents such as oil are applied to the fixing roller, there has arisen another difficulty that part of the toner fixed on the recording material in the first fixing operation is likely to offset in the second fixing operation.
The problems as described above have arisen, when using sharp melting toners, in the fixing apparatus a), b) which adopt the heater type film heating fixing method and the electromagnetic induction film heating fixing method, respectively, in the same way as in the fixing apparatus adopting the heat roller fixing method. In such fixing apparatus, oil must be applied to the film portion in order to part a toner from the fixing film, which has resulted in production of various oil-related problems.
On the other hand, there is a method of solving the above problems at a stroke which uses a toner having wax, as a releasing agent, contained in its binder which contains a coloring material ingredient. According to this method, oil needs not be applied to the fixing members such as fixing roller and fixing film, and occurrence of offsets can be prevented because the wax ingredient oozed from the toner at the fixing nip portion adheres to the surface of the fixing members and forms a releasing layer thereon during the fixing operation.
In recent years, however, with the spread of color printing, not only photo images but also ordinary business documents are more often printed out in color; from this standpoint, there have been increasing demands that the gloss of images should be kept to a minimum. In this respect, there have been demands even for the above toner to be designed in such a manner as to allow the printed image on which the toner has been fixed to be less glossy without decreasing its melt viscosity very much (the sharp melting toner in current use has a low viscosity).
The matters described so far can be put together to form the following viewpoint. As a full color image fixing system for realizing easy image maintenance and satisfactory double-side images while accomplishing on-demand performance as well as energy saving, preferably an image forming apparatus is constructed in such a manner that it includes a fixing apparatus adopting the heater type film heating fixing method or the electromagnetic induction film heating fixing method, like a) or b) described above, and the toner having a releasing agent contained its binder which contains a coloring material ingredient is applied thereto.
Further, as an image characteristic of color images including business documents, preferably the glossiness of the images is not very high and has a moderate gloss value; accordingly, it is necessary not to reduce the melt viscosity of the toner described above very much, but to keep it moderately viscous.
However, when performing fixing operation of the wax-containing toner having characteristics as described above in the above fixing apparatus, as shown in FIG. 16, which adopts the electromagnetic induction film heating fixing method, the following new problems were created.
(1) When the contact pressure between the fixing film 10 and the pressurizing roller 30 was low at the fixing nip portion N, the toner was not melted smoothly, and there occurred a fine mosaic-like image defect known as xe2x80x9cporexe2x80x9d in the images of both middle tone and high density. Particularly in the transparent sheets for use in overhead projectors (OHP), the phenomenon was remarkable and the projected image deteriorated considerably.
(2) When the contact pressure between the fixing film 10 and the pressurizing roller 30 was increased so as to avoid the defect described in (1), there occurred a fixing defect in the form of blistering in the high-density image portion (refer to FIG. 17). Presumably the reason for this defect to occur is that, because a high pressure was applied to the toner in state where its surface was melted, but its interior portion was not completely melted, an anchor effect of the toner on paper (recording material) was not fully produced; therefore, the surface of the toner was pulled on the fixing film side when releasing the toner from the film. Further, when the fixing member was a film, like in this case, there arose a problem that a high contact pressure was likely to cause deterioration and damage to the film.
The phenomena described in (1) and (2) are conflicting with each other, as a result, satisfactory images could not be obtained, and there have been demands for suitable fixing conditions. The above phenomena were also observed in a fixing apparatus adopting the heater type film heating fixing method.
Accordingly, an object of the present invention is to provide an image forming apparatus which starts up quickly and is capable of forming satisfactory images.
Another object of the present invention is to provide an image forming apparatus including an unfixed toner image forming means for forming unfixed toner images and a fixing means for fixing on a recording material the unfixed toner images formed by the unfixed toner image forming means, in which the fixing means includes a film, first and second back up members which sandwich the film therebetween and are in pressure contact with each other to form a nip portion, and a heating means for increasing the temperature of the film, nips and conveys the recording material, bearing the unfixed toner image between the film and the second back up member at the nip portion, and fixes the unfixed toner image on the recording material, the toner of the unfixed toner image contains a releasing agent in a binder which contains a coloring material ingredient and have a melt index value of 0.5 g or more and 100 g or less in accordance with the Melt Index measuring method, and the average pressure at the nip portion is 5.9xc3x97104 Pa or more and 24.5xc3x97104 Pa or less.
Still another object of the present invention is to provide an image forming apparatus including an unfixed toner image forming means for forming unfixed toner images and a fixing means for fixing on a recording material the unfixed toner images formed by the unfixed toner image forming means, in which the fixing means includes a fixing member, a back up member in contact with the fixing member, and a magnetic flux generating means for generating magnetic flux, an eddy is generated in the fixing member by a magnetic flux from the magnetic flux generating means and the fixing means generates heat, the recording material bearing the unfixed toner image is nipped and conveyed at the contact portion between the fixing member and the back up member, thereby the unfixed toner image is fixed on the recording material, the toner of the unfixed toner image contains a releasing agent in a binder which contains a coloring material ingredient and have a melt index value of 0.5 g or more and 100 g or less in accordance with a Melt Index measuring method, and the average surface pressure at the nip portion is 5.9xc3x97104 Pa or more and 24.5xc3x97104 Pa or less.
The foregoing and other objects of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention with reference to the drawings.